DESCRIPTION

This daemon acts as a name server on all hosts involved in
distributed Erlang computations. When an Erlang node
starts, the node has a name and it obtains an address from the host
OS kernel.
The name and the address are sent to the
epmd daemon running on the local host.
In a TCP/IP environment, the address consists
of the IP address and a port number. The name of the node is
an atom on the form of Name@Node.
The job of the epmd daemon is to keep track of which
node name listens on which address. Hence, epmd map
symbolic node names to machine addresses.

The TCP/IP epmd daemon actually only keeps track of
the Name (first) part of an Erlang node name, the Host
part (whatever is after the @ is implicit in the
node name where the epmd daemon was actually contacted,
as is the IP address where the Erlang node can be
reached. Consistent and correct TCP naming services are
therefore required for an Erlang network to function
correctly.

Starting the port mapper daemon

The daemon is started automatically by the erl
command if the node is to be distributed and there is no
running instance present. If automatically launched,
environment variables has to be used to alter the behavior of
the daemon. See the Environment
variables section below.

If the -daemon argument is not given, the
epmd runs as a normal program with the
controlling terminal of the shell in which it is
started. Normally, it should run as a daemon.

Regular start-up options are described in the
Regular options
section below.

These options are available when starting the actual name server. The name server is normally started automatically by the erl command (if not already available), but it can also be started at i.e. system start-up.

-port No

Let this instance of epmd listen to another TCP port than
default 4369. This can also be set using the
ERL_EPMD_PORT environment variable, see the
section Environment
variables below

-d | -debug

Enable debug output. The more -d flags given, the more
debug output you will get (to a certain limit). This option is
most useful when the epmd daemon is not started as a daemon.

-daemon

Start epmd detached from the controlling terminal. Logging will end up in syslog when available and correctly configured. If the epmd daemon is started at boot, this option should definitely be used. It is also used when the erl command automatically starts epmd.

-relaxed_command_check

Start the epmd program with relaxed command checking (mostly for backward compatibility). This affects the following:

With relaxed command checking, the epmd daemon can be killed from the localhost with i.e. epmd -kill even if there are active nodes registered. Normally only daemons with an empty node database can be killed with the epmd -kill command.

The epmd -stop command (and the corresponding messages to epmd, as can be given using erl_interface/ei) is normally always ignored, as it opens up for strange situation when two nodes of the same name can be alive at the same time. A node unregisters itself by just closing the connection to epmd, why the stop command was only intended for use in debugging situations.

With relaxed command checking enabled, you can forcibly unregister live nodes.

Relaxed command checking can also be enabled by setting the environment variable ERL_EPMD_RELAXED_COMMAND_CHECK prior to starting epmd.

Only use relaxed command checking on systems with very limited interactive usage.

These options make epmd run as an interactive command displaying the results of sending queries ta an already running instance of epmd. The epmd contacted is always on the local node, but the -port option can be used to select between instances if several are running using different port on the host.

-port No

Contacts the epmd listening on the given TCP port number
(default 4369). This can also be set using the
ERL_EPMD_PORT environment variable, see the
section Environment
variables below

-names

List names registered with the currently running epmd

-kill

Kill the currently running epmd.

Killing the running epmd is only allowed if epmd
-names show an empty database or
-relaxed_command_check was given when the running
instance of epmd was started. Note that
-relaxed_command_check is given when starting the
daemon that is to accept killing when it has live nodes
registered. When running epmd interactively,
-relaxed_command_check has no effect. A daemon that is
started without relaxed command checking has to be killed
using i.e. signals or some other OS specific method if it has
active clients registered.

-stop Name

Forcibly unregister a live node from epmd's database

This command can only be used when contacting epmd
instances started with the -relaxed_command_check
flag. Note that relaxed command checking has to be enabled for
the epmd daemon contacted, When running epmd
interactively,
-relaxed_command_check has no effect.

This environment variable can contain the port number epmd will use.
The default port will work fine in most cases. A different port can
be specified to allow several instances of epmd, representing
independent clusters of nodes, to co-exist on the same host.
All nodes in a cluster must use the same epmd port number.

ERL_EPMD_RELAXED_COMMAND_CHECK

If set prior to start, the epmd daemon will behave
as if the -relaxed_command_check option was given at
start-up. If consequently setting this option before starting
the Erlang virtual machine, the automatically started
epmd will accept the -kill and -stop
commands without restrictions.

The epmd daemon accepts messages from both localhost and
remote hosts. However, only the query commands are answered (and
acted upon) if the query comes from a remote host. It is always an
error to try to register a nodename if the client is not a process
located on the same host as the epmd instance is running on,
why such requests are considered hostile and the connection is
immediately closed.

The queries accepted from remote nodes are:

Port queries - i.e. on which port does the node with a given
name listen